1. Field of the Invention
This invention relates in general to grinding devices and in particular to a new and useful method and apparatus for improving the grinding result of a pressure chamber grinder.
The present invention particularly concerned with a method and an apparatus for improving the grinding result of a pressure chamber grinder. A pressure chamber grinder is described in U.S. Pat. No. 4,586,661. Therein the finely divided material to be ground is fed by means of a mechanical feeder device into a pressurized equalizing tank, the material, which may have been clotted, is made loose by means of a rotor in the tank, and the material that is made loose in this way is transferred into a pre-grinder. There, several grinding-gas jets are applied to the material to be ground so that the material to be ground is fluidized. The fluidized material-gas flow is passed into a bisecting device, wherein it is divided into two component flows of substantially equivalent magnitude and composition. Each component flow is passed into the main grinding chamber through its own long accelerating nozzle, which nozzles are directed so that a collision zone for the two component flows is formed in the center point of the main grinding chamber.
It is an advantage of such a pressure chamber grinder that, it is operationally economically far superior to conventional jet grinders, wherein ejectors are usually used as a feeder device.
Since in principle, in a pressure chamber grinder, the material particles to be ground are subjected to the grinding effect only once, as a rule, depending on the material to be ground, a very little proportion of the particles can pass through or by-pass the grinding zone without being crushed. Even though the proportion of this coarser material fraction in the whole material flow is, as a rule, very little, e.g. less than 1 per cent by weight, in the case of many products there is a necessity to remove these coarse particles from the ground product. In such a case, it is necessary to resort to a separate classifier, from which the coarse particles are returned, in one way or another, into the main grinding chamber for regrinding.
In practice, it has, however, been noticed that when an extremely finely divided final product is aimed at, such as in the preparation of pigments, a qualitatively and/or economically fully satisfactory final result cannot be achieved by means of any known classifier in use. This is due to the fact that the particle size of the material to be classified is at the maximum a few microns. For example, the primary crystal size of titanium dioxide pigments is of the order of 0.2 micro-meters, and the average particles size of finely divided titanium dioxide pigment grades is only slightly larger than that.
In the jet grinders in common use, in particular in the so-called disk-jet grinders, one of which is described, e.g., in the U.S. Pat. No. 2,032,827, a gas suspension of solid material ends up in a circularoty movement, whereby the centrifugal force prevents the coarse particles from escaping from the grinder until they have been ground sufficiently finely. Further developments of this basic jet grinder are described in several patents, e.g. in the U.S. Pat. No. 3,178,121. Attempts have been made to improve the ability of the basic grinder to classify and to grind the coarser and less readily grindable material fraction included in the material to be ground by to the basic grinder connecting various supplementary grinders and circulation systems for coarse material. Such methods and systems are described, e.g., in the U.S. Pat. Nos. 4,189,102 and 4,248,387. The improvements have given increased efficiency for the grinding of the coarse material, but the solutions are not energy-economically satisfactory. In many cases, the consumption of energy has been increased further. After the apparatuses have become even more complicated, their reliability in operation has suffered at the same time, in particular in the most extensive fine-grindings (pigments), because the narrow pipe systems and uneven flows result in rapid clogging of the equipment. With reduced homogeneity of the gas suspension of the solid material subject of grinding, the ability of classification of the grinder equipments has been deteriorated even if the grinding capacity has been increased. This is seen as a necessity to separate the unground fraction in order to return it to the primary grinding.